1. Field of the Invention
This invention relates to a signal reproducing apparatus for optically reproducing signals recorded on a magnetic recording medium.
2. Description of the Prior Art
Recently, apparatuses for optically reproducing magnetically recorded information by utilization of a vertical magnetic recording medium have been developed. In such apparatuses, reproduction of the information is accomplished by causing a polarized light to enter the recording medium, causing the light beam having its plane of polarization rotated by the Kerr effect or the Farady effect to pass through an analyzer and detecting the light beam by a photodetector. An example of such photomagnetic information reproducing apparatus according to the prior art is shown in FIG. 1 of the accompanying drawings.
In FIG. 1, reference numeral 1 designates a recording medium comprising a photo-magnetic material such as MnBi, TbFeO.sub.3, GdIG, DgCO, GdFe, TbFe, GdTbFe or TbDyFe deposited on a substrate of glass or plastics by vapor deposition or sputtering. The recording medium has information recorded thereon by variations in the direction of magnetization (upward direction or downward direction) and is rotated by a motor 2.
A light emitted from a light source 3 such as a semiconductor laser is collimated into a parallel light beam by a collimater lens 4. A polarizer 5 makes the light beam into a linearly polarized light and causes it to enter the recording medium 1. Designated by 6 is a beam splitter for separating the incident light and the reflected light from the recording medium. The incident light beam is made into a minute spot by a condenser lens 7 and impinges on the recording medium 1, and is reflected with the plane of polarization thereof rotated through .theta.k or -.theta.k by the Kerr effect in correspondence with the direction of magnetization of the recording medium. This reflected light is reflected by the beam splitter 6 and split into two directions by a half-mirror 8. The split light beams respectively pass through analyzers 9 and 10 and condenser lenses 11 and 12 and are directed to photo-detectors 13 and 14, whereby they are are converted to electrical signals.
Such two electrical signals are differentiated by a differential amplifier 15 and utilized as an information reproducing signal.
The reproducing signal is obtained by the differential detection method, which will hereinafter be described in detail by reference to FIGS. 2(A), (B), (C), (D) and (E) of the accompanying drawings.
FIGS. 2(A) and (B) illustrate the directions of the transmission axes of the analyzers 9 and 10. In these Figures, reference numeral 16 designates the direction of the plane of polarization of the light beam entering the recording medium, and reference numerals 17 and 18 denote the planes of polarization of the reflected light beams rotated through -.theta.k and +.theta.k, respectively, by the direction of magnetization of the recording medium.
Now, the transmission axis of the analyzer 9 in FIG. 1 is disposed perpendicularly to the plane of polarization 17 when rotated through -.theta.k, as shown at 19 in FIG. 2(A). On the other hand, the transmission axis of the analyzer 10 of FIG. 1 is disposed perpendicularly to the plane of polarization 18 rotated through +.theta.k, as shown at 20 in FIG. 2(B).
By such an arrangement, the electrical signals obtained from photodetectors 13 and 14 in FIG. 1 become signals which are 180.degree. out of plase with each other (that is, which have been inverted from negative to positive). Usually, the recording medium has an irregular reflection factor, pin-holes, dust, etc. and the noise component resulting therefrom is likewise detected by the detectors 13 and 14 irrespective of the presence of the analyzer. For example, where the recording medium has an irregular reflection factor, the signal waveform undulates due to the noise component as indicated by broken line in FIG. 2(C) or (D). In the prior art example shown in FIG. 1, the differential between the detection signals shown in FIGS. 2(C) and (D) is taken, whereby the noise components which are of the same phase are offset and the information signals are in an added form. Thus, the final reproducing signal is obtained as shown in FIG. 2(E) and accordingly, the S/N ratio of information reproduction is improved.
However, in the conventional photo-magnetic reproducing apparatus as described above, the angle of rotation of the plane of polarization is very small and therefore, in order that the polarized component having information may be separated from the modulated light beam, two expensive analyzers having a very high analyzing performance must be aligned and arranged with high accuracy, and this has led to cumbersome adjustment and high cost.
Also, for efficient information reproduction, the incident light must be properly spotted on the recording medium, and in the photo-magnetic reproducing apparatus as described above, focus control has been accomplished by the use of a well-known focusing method and by a mechanism, not shown, in accordance with the control signal from the detector 13 or 14. However, the light beams received by the detectors 13 and 14 have passed through the analyzers 9 and 10, respectively. The analyzers decrease the quantity of light passed therethrough, and this has led to the disadvantage that focus control of high accuracy cannot be achieved.